Thanks for your response. This is a fully licensed font - ITC Kabel Std. I did validate the font and found that somehow two copies had been installed in the Fonts folder by accident. I deleted the duplicates and restarted Motion but the problem is still there...

I've seen this problem several times and, for me, it was always a case of scaling the wrong thing.

You probably scaled the video size to bring down the size of the font, which closes in the bounding box when you should leave the video size alone and scale WITHIN the bounding box by scaling the text.